linux-kernel - Re: [PATCH v3 09/13] sched/fair: Let asymmetric cpu configurations balance at wake-up

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Message-ID: <20160815133949.GT6879@twins.programming.kicks-ass.net>
Date:	Mon, 15 Aug 2016 15:39:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	mingo@...hat.com, dietmar.eggemann@....com, yuyang.du@...el.com,
	vincent.guittot@...aro.org, mgalbraith@...e.de,
	sgurrappadi@...dia.com, freedom.tan@...iatek.com,
	keita.kobayashi.ym@...esas.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 09/13] sched/fair: Let asymmetric cpu configurations
 balance at wake-up

On Mon, Jul 25, 2016 at 02:34:26PM +0100, Morten Rasmussen wrote:


Because I forgot _again_, I added:

/*
 * Disable WAKE_AFFINE in the case where task @p doesn't fit in the
 * capacity of either the waking CPU @cpu or the previous CPU @prev_cpu.
 *
 * In that case WAKE_AFFINE doesn't make sense and we'll let
 * BALANCE_WAKE sort things out.
 */

> +static int wake_cap(struct task_struct *p, int cpu, int prev_cpu)
> +{
> +	long min_cap, max_cap;
> +
> +	min_cap = min(capacity_orig_of(prev_cpu), capacity_orig_of(cpu));
> +	max_cap = cpu_rq(cpu)->rd->max_cpu_capacity;

There's a tiny hole here, which I'm fairly sure we don't care about. If
@p last ran on @prev_cpu before @prev_cpu was split from @rd this
doesn't 'work' right.

> +	/* Minimum capacity is close to max, no need to abort wake_affine */
> +	if (max_cap - min_cap < max_cap >> 3)
> +		return 0;
> +
> +	return min_cap * 1024 < task_util(p) * capacity_margin;
> +}
> +
>  /*
>   * select_task_rq_fair: Select target runqueue for the waking task in domains
>   * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE,
> @@ -5389,7 +5414,8 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
>  
>  	if (sd_flag & SD_BALANCE_WAKE) {
>  		record_wakee(p);
> -		want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, tsk_cpus_allowed(p));
> +		want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu)
> +			      && cpumask_test_cpu(cpu, tsk_cpus_allowed(p));
>  	}
>  
>  	rcu_read_lock();
> -- 
> 1.9.1
>